A starter relay, or starter solenoid, works by completing the circuit that brings electric current from a vehicle's battery to the starter motor of the engine. The starter relay can typically be located by following the red wire from the battery to the relay.
All solenoids are simple electromagnets consisting of a coil and a spring-loaded iron armature. When a current passes through the coil of a solenoid, the armature moves to increase the flux. When the current is switched off, the armature contracts.
In a starter solenoid, when a key is turned in the car ignition, the movement of the armature closes the pair of heavy contacts that serve as the bridge between the battery and the starter motor. To ensure the starter relay functions properly, it must receive sufficient power from the battery. Insufficiently charged batteries, corroded connections and damaged battery cables can all prevent the starter relay from receiving enough power to operate correctly. When this happens, an audible clicking noise is typically heard when the ignition key is turned. Because it contains moving parts, the starter relay itself can also fail over time. If it fails, the ignition makes no sound when the ignition key is turned.